The invention is directed to a method of reducing the dust behavior of silicates. The invention is also directed to a silicate having reduced dust behavior.
Silicates, particularly needle-shaped silicates such as wollastonite, which is a chain silicate, are being employed as reinforcing fillers in plastic uses.
Wollastonite is a natural calcium silicate of formula Ca3[Si3O9]. Monoclinic wollastonite is comprised of SiO4 tetrahedrons linked in a three-membered arrangement of formula [Si3O9]6xe2x88x92wherein the individual chains are linked via Ca2+ions. This explains why wollastonite particles may have a needle-shaped structure.
In addition, wollastonite has beneficial material properties, including the absence of chemically bound water which in other hydrous silicates gives rise to changes in structure by loss of water upon heating. When incorporated in plastic materials, the result would be that a reinforcing effect no longer could be achieved. Further advantages are a low degree of shrinkage upon drying and heating, an exceedingly low expansion coefficient, and high mechanical properties of manufactured plastic products including wollastonite. In addition to its use as an additive in plastic materials, wollastonite is also used in the manufacture of ceramic products, sanitary articles, and porcelain.
The present invention predominantly relates to the use of wollastonite in plastic materials.
In such uses, wollastonite as a filling material is produced by grinding recovered wollastonite, followed by air classification. As a result, needle-shaped wollastonite particles are obtained, which either are incorporated directly in the plastic mixture or incorporated in the plastic materials in the form of polymer masterbatches including levels of from 10 to 40 wt.-% of wollastonite. Owing to the needle-shape structure of wollastonite, a substantial improvement in the mechanical properties is achieved in the plastic material and plastic molded articles produced therefrom.
In addition, according to the prior art, wollastonite frequently is coated with silane compounds in order to achieve improved incorporation in the plastic material. To this end, surface coating using aminosilane, epoxysilane, methacrylic silane, trimethylsilane, vinylsilane, or alkylsilane is performed.
However, these prior art silicate materials involve the disadvantage of exhibiting increased dust formation due to an increased dispersity as a result of silane coating. The dust portion is comprised of most finely particulate silicate particles essentially lacking the needle-shape structure and therefore being incapable of inducing any reinforcing effects in the plastic material. That is, part of the silicate filler is not effective, acting merely as a non-reinforcing powdered filling material.
Initially, attempts have been made to overcome this drawback by subjecting wollastonite to various procedures in order to decrease the dust behavior. However, such procedures are relatively costly and expensive, thereby resulting in a substantial rise in cost of this raw material, rendering it useless as reinforcing filler in plastic materials for cost reasons. Therefore, this raw material cannot compete with other raw materials such as fiber glass or other fibrous materials also used as reinforcing fillers.
It was therefore the technical object of the invention to provide a simple method of reducing the dust behavior of silicates, particularly of wollastonite, which method would be practicable at a cost as low as possible and result in a substantial decrease in the dust behavior of wollastonite.
Said technical object is accomplished by subjecting the comminuted silicate, particularly wollastonite, to a surface coating using a polyorganosiloxane.
In contrast to previous surface coating using silanes, coating using polyorganosiloxanes was found to result in a substantial reduction of the dust behavior of much more than 50% as compared to prior art materials. Surface coating of silicates is a simple, cost-effective process. Thus, the appropriate surface coating agents normally are coated onto the surface by mixing the material in a fluid mixer. Furthermore, this technique has become well-tried in coating using appropriate silane compounds owing to this simple method of reducing the dust behavior of silicates, the previous multi-stage physical processes of reducing the dust behavior are no longer necessary and can be replaced in a simple fashion.
In particular, it has been surprising that the dust behavior in surface coating using appropriate silane compounds is not reduced or only to a minor degree, while coating using polyorganosiloxanes results in a significant reduction of the dust behavior. In this context, adequate comparative experiments were carried out which will be described in the experimental section of this application.
In a particularly preferred embodiment, a polyorganosiloxane in an amount of from 0.1 to 4 wt.-%, more preferably from 0.5 to 1.5 wt.-% is used in surface coating. A polysiloxane-polyether copolymer is employed as a particularly preferred polyorganosiloxane.
As has been mentioned above, these polyorganosiloxanes allow for a reduction of the dust behavior of silicates, particularly of calcium metasilicates, and in a particularly preferred fashion, of wollastonites.
Surprisingly, this dust-reducing surface coating was found to achieve good incorporation of the filler in the plastic material. By using the polyorganosiloxane compounds of the invention, the same effect is achieved as with the corresponding functional silane compounds known from prior art.
The invention is also directed to a silicate having reduced dust behavior, which silicate is surface-coated with a polyorganosiloxane, has a Heubach dust content of from 500 to 1200 mg/100 g, the silicate having a specific surface area of from 0.5 to 1.5 m2/g, as measured according to DIN 66132-BET. The silicate preferably is a calcium metasilicate, and more preferably a wollastonite. The silicate preferably is surface-coated using from 0.1 to 4 wt.-%, more preferably from 0.5 to 1.5 wt.-% of polyorganosiloxane. A polysiloxane-polyether copolymer preferably is employed as polyorganosiloxane.
The silicate is used as a reinforcing filler in plastics, particularly in polyurethane, polyamide, and polypropylene. The silicate is produced by grinding the wollastonite on an appropriate size reduction unit, such as a counterflow impact grinding unit. The resulting comminuted product then is removed from the size reduction unit and surface coated with the polyorganosiloxane in a fluid mixer. Coating is effected at temperatures between 40 and 100xc2x0 C. resulting from frictional heat. Following coating, the material is cooled to room temperature. A low-dust coated wollastonite having the above-mentioned properties is obtained as final product.
The product according to the invention will be characterized in more detail in the Figures which follow.